In the following, a channel resource means a frequency bandwith, a time interval and possibly several spatial dimensions brought by transmit and receive antennas placed in different spatial positions.
Signals transmitted over wireless channels undergo severe degradations due to channel variations such as fading, shadowing, and interference from other transmitters, which allows considering the channel as a random variable. In the following, are considered slow channel variations with respect to the time needed for transmission of the information words, but the channel realization is supposed to have changed between two transmissions of information words. One major way to combat the so-called quasi-static fading is to provide diversity in either time, frequency, or space.
The channel diversity order is defined as the number of independent fading random variables observed in the channel resource. The transmission/reception scheme is able to collect a given amount of diversity, called diversity order of the system, upper bounded by the channel diversity order, also called full diversity order.
When an information word is not correctly estimated by the receiver, an error event occurs. The main parameter allowing to compute the probability of error associated to a given error event is the Euclidean distance between the noiseless received signal associated to the transmitted information word and the reconstructed noiseless received signal associated to the decoded information word. The diversity order of the error event is defined as the number of independent random variables involved in the Euclidean distance associated to the error event. Finally, the system diversity order is equal to the minimal diversity order of all possible error events or equivalently all possible pairs of information words.
Multiple-antenna systems that provide high orders of spatial diversity and high capacity have been extensively studied. However, due to the expensive analog Radio-Frequency components, the number of antennas in mobile terminals is often limited to one transmission Radio-Frequency path and two reception Radio-Frequency paths. Such mobile terminals do not allow transmitted signals to attain high diversity orders over narrow band channels, such as OFDM-based (Orthogonal Frequency-Division Multiplexing) transmission systems.
The concept of ‘cooperative communications’ has then been introduced where multiple terminals use the capabilities of each other. This allows the signals to attain high diversity orders, being transmitted by other terminals leading in that way to a virtual antenna array as proposed by Sendonaris et al. in the article ‘User cooperation diversity. Part 1. system description’, IEEE Transactions on Communications, vol. 51, no. 11, pp. 1927-1938, November 2003’. In the following, we describe the idea of cooperative transmission protocols, which is an application of the invention described hereafter.
One potential application of such systems is in wireless ad-hoc networks, such as mesh networks for instance, which does not depend on a central control unit and does not have a fixed infrastructure. The nodes communicate by forming a network based on channel conditions and mobile locations.
Another application of such systems is the cooperation of in-cell users in a cellular system. Reliable communication can be achieved through diversity and by relaying signals from terminals that are far from the base station. The advantage of such systems compared to traditional ones is that the more users there are in a network, the more reliably one can communicate. This is the result of the non-rigidity of the infrastructure of cooperative systems. In the opposite, the rigidity of the infrastructure of non-cooperative systems involves an increase of blocking probability with the number of terminals that are sharing the network.
The drawback of cooperative systems is that the inter-user channel is noisy. In order to counterfeit this drawback, multiple cooperation protocols have been developed and define the way the cooperation between users is performed.
Cooperation protocols can be classified into two major categories: Amplify-and-Forward (AF) and decode-and-forward (DF). We focus in the following on the AF protocols.
Using the AF protocol, a relay, which is for example an user mobile radio equipment or an equipment used for extending the range of a cell, only amplifies the signal received from a source (a base station or another relay) before forwarding it to a destination (a base station or another relay). These protocols are easy to implement in practice, as the computational complexity they introduce at the relays is limited to a scaling operation.
Multiple AF protocols have been designed for the single-relay case such as, for example, the non-orthogonal amplify-and-forward (NAF) protocol, also known as the TDMA-based Protocol I, in which the source broadcasts a signal to both the relay and the destination in the first phase. In the second phase, the relay scales the signal and forwards it to the destination, while the source transmits another message to the destination (Nabar et al., “Fading relay channel: performance limits and space-time signal design’, IEEE Journal on Selected Areas in Communications, vol. 22, no. 6, pp. 1099-1109, August 2004).
In the following, we focus on the case where multiple relays are used by the transmission system. The selected transmission protocol transmits a signal over each time slot of a set of predetermined set of M time slots.
For example, the well-known Slotted Amplify-and-Forward (SAF) protocol may be used. However, the invention is not limited to the transmission over a half-duplex non-orthogonal Amplify-and-Forward cooperative channel.